The present invention relates to a control unit of a direct-current motor, and more particularly to a control unit of a direct-current motor suitable for use in a magnetic recording and reproducing device or the like.
A conventional example of a control unit of a direct-current motor in a helical scanning type magnetic recording and reproducing device is shown in FIG. 1 (See "SONY Semiconductor IC Data Book (1989)" pp. 338-339, pp. 361-362 and pp. 376-377). In FIG. 1, a reference numeral 201 represents a drum motor for driving a rotary head, 202 a capstan motor for driving a tape, 203 a pinch roller and 204 a magnetic tape, respectively. The drum motor 201 is fitted with detectors 205 and 206 for detecting a rotational frequency and a rotational phase thereof, respectively. The frequency detector 205 detects a magnetic field change attendant upon the rotation of the drum motor 201 for instance using a stationary coil, and the rotational phase detector 206 detects a magnetic field change caused by a magnet fitted at a specific location of a rotor of the drum motor 201 for instance using a stationary coil.
The rotational frequency signal S.sub.DF and a rotational phase signal S.sub.DP detected by means of the detectors 205 and 206 are fed into waveform shapers 209 and 210 through amplifiers 207 and 208 and converted into rectangular wave signals, and are fed thereafter into a circuit 211 for computing an error of the rotational frequency (the number of rotations) and an error of the rotational phase. The error arithmetic circuit 211 outputs the values of the errors obtained by processing the frequency signal S.sub.DF and the phase signal S.sub.DP as digital signals in m bits.
A digital frequency error signal A.sub.DF and digital phase error data A.sub.DP are converted into analog signals by means of DA converters 212 and 213 and limited in bands by means of low-pass filters 214 and 215, and are added analogically to each other thereafter by an adder 216. An obtained analog error signal S.sub.AD undergoes required compensation of the gain and compensation of the phase by means of an analog amplifier 217 and an analog phase compensator 218, and fed into a control signal generating circuit 220 thereafter through an analog differential amplifier 219.
The control signal generating circuit 220 generates a pulse width modulation signal (a PWM signal) in which the pulse width changes in accordance with the value of the analog error signal S.sub.AD and supplies the relevant signal to a chopper circuit 221 as a control signal S.sub.PWM of the chopper circuit 221. The chopper circuit 221 intermits voltage E.sub.0 supplied from a direct-current power source 224 under the control of the control signal S.sub.PWM, thus generating a motor driving signal S.sub.DR. This driving signal is smoothed by means of a smoothing circuit 222 and supplied to an exciting coil (not shown) of the drum motor 201 thereafter, thereby to control the rotational frequency and the rotational phase of the motor. Besides, the driving signal S.sub.DR after smoothing is fed back to one terminal of the differential amplifier 219 through a feedback circuit 223, and is used as a negative feedback signal for compensating fluctuation of the power source voltage E.sub.0.
Viewed in the light of achieving miniaturization or low cost of a recording and reproducing device, it is desirable that the control unit of the drum motor 201, in particular the processing circuit of error signals (the frequency error signal A.sub.DF and the phase error signal A.sub.DP) is digitized as far as practicable. Because, when it is possible to digitize the circuit, it is possible to omit analog circuit components such as a DA converter and a low-pass filter that cause to make the device larger in size and increase the cost thereof. However, since the generating circuit of the driving signal S.sub.DR composed of the differential amplifier 219, the control signal generating circuit 220, the chopper circuit 221, the smoothing circuit 222 and the feedback circuit functions practically as a negative feedback power amplifier, it is required to convert the digital error signals A.sub.DF and A.sub.DP for making the generating circuit operate into analog signals and feed these analog signals into the differential amplifier 219. As a result, it has been technically difficult to digitize all of the processing circuits of these error signals with the conventional circuit configuration shown in FIG. 1.